I think you're on the right track when you suspect that something like what you describe as a "frequency spike" may well be at the heart of the issue in question.

Let me assure you that the actual signal path from capsule output to mixer input is as linear as a cable when using XD-V70s and V75s, with no inherent "eq" or companding that could affect the subjective balance of your mix.

I make these comments from the perspective of a working Audio Director/Sound Supervisor who's approaching half a century of ongoing mixing and recording experience, most of it live or "as live" ...and one who has neither affiliation with, nor particular loyalty to, Line 6. (Click on my pink avatar for more details.)

My feeling is that you should experiment with Line 6's different mic models before going to the trouble and expense of capsule replacement.

To my ears, the "standard L6" model does exhibit the open and extended high frequency response that (while being much loved by regular users of condenser vocal mics such as Neumann KMS104s and 105s) could well be giving rise to the "out front" effect that you mention.

I'd be very surprised if you could not find a sound that "sits right" for you and would suggest that you start with the silky bass that Line 6 have modeled from Electro-Voice's neodymium-capsuled N/D767a in their "767" setting.

Then have a listen to the "b58", "935" and "d5" models.

I can report that the "b58" has proven to be the most "familiar" to vocalists in just over a year on the road using my V70 handhelds, with many subsequently relishing the ability to "tailor" my handhelds to their particular vocal styles.